3D modelling of unpolarized photon diffusion using the integral form of the transport equation

The 3D scalar transport equation for unpolarized photons is used to give a detailed description of the fluorescence photon diffusion from a homogeneou s slab, As an example, the paper considers, with a complete 3D spatial desc ription in plane geometry, the distribution both in physical and momentum s pace of the primary photons induced by a narrow radiation beam crossing the slab. Then it is shown how the 3D geometry influences the shape of the con tinuous spectra due to a second Compton collision which modifies the distri bution of the primaries due to photoelectric effect. The possibility of iso lating the effect of a particular interaction is one of the strengths of th e multiple-scattering scheme in the framework of transport techniques, whic h allows a better understanding of photon diffusion. In order to evaluate t he effects of boundary conditions, the integral transport equation is used instead of the integro-differential equation, which has the advantage of tr eating the how of photons from the outer space as an external source. The r esults are compared with those obtained for a half-infinite medium uniforml y irradiated with a plane infinite slant source of monochromatic photons pr eviously solved in 1D, Copyright (C) 1999 John Whey & Sons, Ltd.